Reverse circulation drilling with dual concentric drill pipe has become an accepted technique, with many advantages over conventional drilling. A system for reverse circulation dual tube drilling is disclosed in Henderson U.S. Pat. No. 3,208,539. The advantages of this technique include virtual elimination of lost-circulation problems, reduction in power and drilling fluid needs, virtually complete recovery of uncontaminated cuttings, and the capability for continuous core recovery.
In reverse circulation dual tube drilling systems, the drilling fluid is pumped from the surface to the bit through an annular space defined between the inner and outer concentric drill pipes. The fluid, with entrained cuttings returns to the surface through the inner pipe.
In such systems, it becomes important in many drilling applications to insure that the drilling fluid is forced to return to the surface through the inner pipe, and steps must be taken to prevent the fluid from passing upward in the annular space between the outer pipe and the hole wall. Several techniques have been developed in an attempt to accomplish this purpose. For example, heavy static fluids have been placed in the hole annulus to prevent upward flow of drilling fluid therein. Mechanical means, such as shrouded or skirted bits and bit subs are disclosed in Henderson U.S. Pat. No. 2,819,043, Elenburg U.S. Pat. No. 3,439,757, and Elenburg U.S. Reissue Pat. No. 27,316. These mechanical structures suffer several disadvantages. First, they rotate with the drill string and are consequently abraded by the hole wall, resulting in shortened life. Second, they must necessarily be of the same or slightly less diameter than the effective cutting diameter of the bit, thus inherently resulting in an imperfect seal. Finally, they are subject to sticking and lodging in the hole due to the passage of small cuttings from the hole bottom and the accumulation of detritus from above.